Encoded video naturally has variable bit-rate (VBR), as the number of bits produced when encoding a picture depends on the picture content: how similar it is to previously encoded pictures and how much detail it contains. Some video scenes can be coded to a given quality with a small number of bits, whereas other scenes may require significantly more bits to achieve the same quality. When constant bit-rate (CBR) encoding is used, video has to be coded at time varying quality to meet the bit-rate constraint. This has been shown to be sub-optimal to the user, who would prefer to see constant quality. Also, by fixing the bit-rate independent of the genre of the video content, some genres of content can be encoded well, such as news and drama, whereas others, such as fast moving sport and music videos and concerts, can only be coded quite poorly.
The overall perceptual quality of a video sequence is not (generally) the mean of the quality of individual small chunks of the overall video sequence, but is biased towards the worst quality observed during the overall video sequence or parts thereof. Hence by encoding with near-constant quality (as opposed to encoding with a constant bit-rate sufficiently high to ensure that all scenes are encoded so as to be at or above a particular perceptual quality level), bits are not wasted encoding some scenes to a higher quality than necessary given the quality at which other scenes may be encoded, and a smaller overall encoded file size can be achieved compared to using constant bit-rate encoding.
Encoding with constant perceptual quality is, however, not widely supported by commercial video encoding platforms, which generally support constant bit-rate encoding.